Phorbol ester activation of the rat vascular myocyte Na(+)-H(+) exchanger isoform 1

Na+/H+ exchanger and cardiac hypertrophy

Reactive cardiac hypertrophy (CH) is an increase in heart mass in response to hemodynamic overload. Exercise-induced CH emerges as an adaptive response with improved cardiac function, in contrast to pathological CH that represents a risk factor for cardiovascular health. The Na⁺/H⁺ exchanger (NHE-1) is a membrane transporter that not only regulates intracellular pH but also intracellular Na⁺ concentration. In the scenario of cardiovascular diseases, myocardial NHE-1 is activated by a variety of stimuli, such as neurohumoral factors and mechanical stress, leading to intracellular Na⁺ overload and activation of prohypertrophic cascades. NHE-1 hyperactivity is intimately linked to heart diseases, including ischemia-reperfusion injury, maladaptive CH and heart failure. In this review, we will present evidence to support that the NHE-1 hyperactivity constitutes a "switch on/off" for the pathological phenotype during CH development. We will also discuss some classical and novel strategies to avoid NHE-1 hyperactivity, and that are therefore worthwhile to improve cardiovascular health.

Intracellular pH in the resistance vasculature: regulation and functional implications

Net acid extrusion from vascular smooth muscle (VSMCs) and endothelial cells (ECs) in the wall of resistance arteries is mediated by the Na(+),HCO(3)(-) cotransporter NBCn1 (SLC4A7) and the Na(+)/H(+) exchanger NHE1 (SLC9A1) and is essential for intracellular pH (pH(i)) control. Experimental evidence suggests that the pH(i) of VSMCs and ECs modulates both vasocontractile and vasodilatory functions in resistance arteries with implications for blood pressure regulation. The connection between disturbed pH(i) and altered cardiovascular function has been substantiated by a genome-wide association study showing a link between NBCn1 and human hypertension. On this basis, we here review the current evidence regarding (a) molecular mechanisms involved in pH(i) control in VSMCs and ECs of resistance arteries at rest and during contractions, (b) implications of disturbed pH(i) for resistance artery function, and (c) involvement of disturbed pH(i) in the pathogenesis of vascular disease. The current evidence clearly implies that pH(i) of VSMCs and ECs modulates vascular function and suggests that disturbed pH(i) either consequent to disturbed regulation or due to metabolic challenges needs to be taken into consideration as a mechanistic component of artery dysfunction and disturbed blood pressure regulation.

The role of NHE-1 in myocardial hypertrophy and remodelling

Na-H exchange (NHE) is the primary process by which the cardiac cell extrudes protons particularly under conditions of intracellular acidosis. Nine isoforms of NHE have now been identified. Although these antiporters are expressed in virtually all tissues, cardiac cells posses primarily the ubiquitous NHE-1 subtype. It has been well established that NHE-1 is a major contributor to acute ischemic and reperfusion injury although it is now emerging that NHE-1 contributes to chronic maladaptive myocardial responses to injury such as post-infarction myocardial remodelling and likely contributes to the development of heart failure. Experimental studies using both in vitro approaches as well as animal models of heart failure have consistently demonstrated a beneficial effect of NHE-1 inhibitors in attenuating hypertrophy in response to various stimuli as well as inhibiting heart failure in a variety of animal models representing experimentally-induced or genetic models of heart failure. The beneficial effects of NHE-1 inhibitors occur independently of infarct size reduction or on any direct effects on afterload thus implicating a direct antiremodelling influence of these agents. It is proposed that NHE-1 inhibition represents a potentially effective new therapeutic approach for the treatment of heart failure.

Intracellular pH as a determinant of vascular smooth muscle function

Intracellular pH (pHi) is a physiological parameter that is intimately linked to contractility, growth and proliferation of vascular smooth muscle (VSM). Regarding contractility, no general unifying concept of pHi regulation but a rather complex relation between pHi signals and vascular tone has been revealed so far. The modulation of vasotone by pHi depends on the type of blood vessel as well as on the pattern of regulatory input signals. In addition, changes in pHi have been recognized as an important cellular signal to determine the fate of cells in terms of proliferation or apoptosis. Cellular sensors for pHi include a variety of ion transport systems which control intracellular Ca2+ gradients and are likely to serve as a link between pHi and cell functions. Here we provide an overview on the potential targets and mechanisms that transduce pHi signals in VSM. The role of pHi-sensing signaling complexes and localized pHi signaling as the basis of diversity of pHi regulation of VSM function is discussed.

Effects of [5-(2-methoxy-5-fluorophenyl)furan-2-ylcarbonyl]guanidine (KR-32560), a novel sodium/hydrogen exchanger-1 inhibitor, on myocardial infarct size and ventricular arrhythmias in a rat model of ischemia/reperfusion heart injury

The cardioprotective effects of the novel sodium/hydrogen exchanger-1 (NHE-1) inhibitor KR-32560 {[5-(2-methoxy-5-fluorophenyl)furan-2-ylcarbonyl]guanidine} were studied in an anesthetized rat model of 30-min ischemia / 2.5-h reperfusion heart injury. KR-32560 (0.01 - 1 microM) dose-dependently inhibited NHE-1-mediated rabbit platelet swelling induced by intracellular acidification. KR-32560 at 0.1 and 1.0 mg/kg (i.v. bolus, given 10 min before ischemia) reduced infarct size from 65.9% (control) to 49.7% and 32.7%, respectively, while reducing the extension of myocardial injury (mm(3)/g of left heart weight) from 405.1 (control) to 302.9 and 185.4, respectively (all P<0.05 vs control). KR-32560 dose-dependently reduced the total number of ventricular premature beats (VPBs) during ischemia from 510.2 (control) to 353.8 and 134.2 beats (all P<0.05, n = 6), while reducing ventricular tachycardia (VT) incidence from 49.3 (control) to 26.8 and 4.3 and VT duration from 249.2 s (control) to 150.5 and 26.7 s (all P<0.05, n = 6). KR-32560 dose-dependently reduced ventricular fibrillation (VF) incidence from 19.0 (control) to 9.2 and 1.2 and VF duration from 88.0 s to 34.5 and 2.8 s (all P<0.05, n = 6). KR-32560 also exerted similar effects on reperfusion arrhythmias, except for VPBs. These results indicate that KR-32560 may exert significant cardioprotective effects in ischemia/reperfusion heart injury.

High glucose concentrations stimulate human monocyte sodium/hydrogen exchanger activity and modulate atherosclerosis-related functions

In the present study, the effect of high (20 mM) glucose concentrations on human monocyte sodium/hydrogen exchanger (NHE1) activity, scavenger receptor CD36 expression, cell adhesion, and cell migration have been investigated. Incubation with high glucose concentrations caused an increase in NHE1 activity, as estimated by internal pH and sodium-uptake measurements. This effect was specific for glucose, since it was not observed when monocytes were incubated in the presence of 20 mM of galactose, fructose, or mannitol. In addition, the activation of sodium uptake was inhibited by ethylisopropyl amiloride (EIPA), phloretine and cytochalasine B, and calphostin C. High glucose concentrations also increased the expression of CD36 receptors on the surface of monocytes and positively influenced monocyte migration and adhesion to laminin. EIPA added together with glucose counteracted these effects. The data of the present study suggest that a high glucose concentration can influence atherosclerosis-related monocyte functions via NHE1 activation.

The 1.4-MDa apoptosome is a critical intermediate in apoptosome maturation

Previously, we demonstrated that both 150 mM KCl and alkaline pH inhibit cytochrome c-mediated activation of procaspase-3 in a unique manner. To determine the mechanism of inhibition, we analyzed the effect of KCl and alkaline pH on the formation of apoptosomes (a large complex consisting of cytochrome c, Apaf-1, and procaspase-9/caspase-9) in vitro. Our results suggest that an initial ∼700-kDa apoptosome matures through a 1.4-MDa intermediate before a ∼700-kDa apoptosome is reformed and procaspase-3 is activated. We further demonstrate that 150 mM KCl interferes with the conversion of the initial ∼700-kDa apoptosome to the 1.4-MDa intermediate, while alkaline pH “traps” the apoptosome in the 1.4-MDa intermediate. Analysis of the cleaved state of procaspase-9 and procaspase-3 suggests that the 1.4-MDa intermediate may be required for cleavage of procaspase-9. Consistent with these results, in vivo data suggest that blocking acidification during the induction of apoptosis inhibits activation of procaspase-3. On the basis of these results, we propose a model of apoptosome maturation.

Diverse effects of monensin on capacitative Ca2+ entry and release of stored Ca2+ in vascular smooth muscle cells

The effects of monensin, an activator of Na(+)/H(+) exchanger (NHE), on capacitative Ca(2+) entry (CCE) were investigated using A7r5 cells. Capacitative Ca(2+) entry was induced by elevation of extracellular Ca(2+) concentrations of A7r5 cells in which stored Ca(2+) had been depleted by previous administration of thapsigargin. Capacitative Ca(2+) entry was abolished by pretreatment of the cells with SKF-96365 (1-[beta-(3-[4-methoxyphenyl]propoxy)-4-methoxyphenethyl]-1H-imidazole hydrochloride) but was not affected by pretreatment with verapamil. Monensin significantly increased capacitative Ca(2+) entry. On the other hand, 5-hydroxytryptamine-induced inositol monophosphate accumulation and subsequent intracellular Ca(2+) release from its stores were significantly inhibited by monensin, while thapsigargin-induced Ca(2+) release was not affected by monensin. These results suggest that monensin has diverse actions on capacitative Ca(2+) entry and agonist-induced release of stored Ca(2+) in vascular smooth muscle cells.

Targeting the myocardial sodium-hydrogen exchange for treatment of heart failure

Although the past number of years have seen a substantial improvement in the therapeutic approaches for the treatment of heart failure, mortality rates continue to be high. Moreover, the incidence of heart failure is expanding rapidly. Sodium-hydrogen exchange (NHE) is a key target for the treatment of heart failure. NHE is a major mechanism for intracellular pH regulation in most cell types, including the cardiac cell. Seven isoforms of NHE have been identified so far although cardiac cells possess primarily the ubiquitous NHE-1 subtype. NHE-1 is a major contributor to ischaemic and reperfusion injury and NHE-1 inhibitors exert marked cardioprotective effects, particularly when administered before ischaemia, findings which have now been extended to clinical trials. It is emerging that NHE-1 also contributes to chronic maladaptive myocardial responses to injury (myocardial remodelling) and may contribute to the development of heart failure. Experimental studies using both in vitro approaches as well as animal models of heart failure have consistently demonstrated a beneficial effect of NHE-1 inhibitors in terms of inhibition of hypertrophy in response to various stimuli as well as inhibiting heart failure after coronary artery ligation. These effects occurred independently of any infarct size reducing effects of NHE-1 inhibitors or on any direct effects on afterload thus indicating a direct effect on the myocardial remodelling process. In fact, it appears that NHE-1 may represent a common downstream mediator for various hypertropic factors such as angiotensin II, endothelin-1 and beta(1) adrenergic receptor activation. NHE-1 inhibition, therefore, represents a potentially effective new therapeutic approach for the treatment of heart failure.

Inhibition of Na+/H+ exchanger reduces infarct volume of focal cerebral ischemia in rats

Activation of Na+/H+ exchanger (NHE) may have an important role in ischemic cell death by means of intracellular overload of Na(+) and Ca(2+). Recent evidence has suggested that inhibitors of NHE have protective effects on myocardial ischemia both in vivo and in vitro. In this study, we tested the hypothesis that FR183998, an inhibitor of NHE, reduces infarct volume produced by focal cerebral ischemia in rats. We used 20 male spontaneously hypertensive rats. Either FR183998 (1 mg/kg; n=10), or vehicle (n=10) was given intravenously to the rats and the distal middle cerebral artery of each animal was occluded using a photothrombotic technique. We measured regional cerebral blood flow using laser-Doppler flowmetry throughout the experiments. After 3 days, infarct volume was measured in each animal group. To estimate the brain edema, we also calculated the cortical volume in both hemispheres. The infarct volume in the FR183998-treated group (82+/-8 mm(3), mean+/-S.E.M.) was significantly smaller than that in the control group (115+/-12 mm(3)) (P=0.034). The cortical volume of the occluded side in the FR183998-treated group (359+/-7 mm(3)) tended to be smaller than that in the control group (378+/-9 mm(3)) (P=0.116). The regional cerebral blood flow and physiological variables during ischemia were not significantly different between the two groups throughout the experiments. These results suggest that inhibition of NHE by FR183998 may have beneficial effects in reducing infarct volume and brain edema during cerebral ischemia. Thus, NHE may play an important role in the development of neuronal damage during acute cerebral ischemia.

The myocardial Na+/H+ exchanger: a potential therapeutic target for the prevention of myocardial ischaemic and reperfusion injury and attenuation of postinfarction heart failure

The myocardial Na+/H+ exchange (NHE) represents a major mechanism for pH regulation during normal physiological processes but especially during ischaemia and early reperfusion. However, there is now very compelling evidence that its activation contributes to paradoxical induction of cell injury. The mechanism for this most probably reflects the fact that activation of the exchanger is closely coupled to Na+ influx and therefore to elevation in intracellular Ca2+ concentrations through the Na+/Ca2+ exchange. The NHE is exquisitely sensitive to intracellular acidosis; however, other factors can also exhibit stimulatory effects via phosphorylation-dependent processes. These generally represent various autocrine and paracrine as well as hormonal factors such as endothelin-1, angiotensin II and alpha1-adrenoceptor agonists, which probably act through receptor-signal transduction processes. Thus far, 6 NHE isoforms have been identified and designated as NHE1 through NHE6. All except NHE6, which is located intracellularly, are restricted to the sarcolemmal membrane. In the mammalian myocardium the NHE1 subtype is the predominant isoform, although NHE6 has also been identified in the heart. The predominance of NHE1 in the myocardium is of some importance since, as discussed in this review, pharmacological development of NHE inhibitors for cardiac therapeutics has concentrated specifically on those agents which are selective for NHE1. These agents, as well as the earlier nonspecific amiloride derivatives have now been extensively demonstrated to possess excellent cardioprotective properties, which appear to be superior to other strategies, including the extensively studied phenomenon of ischaemic preconditioning. Moreover, the salutary effects of NHE inhibitors have been demonstrated using a variety of experimental models as well as animal species suggesting that the role of the NHE in mediating injury is not species specific. The success of NHE inhibitors in experimental studies has led to clinical trials for the evaluation of these agents in high risk patients with coronary artery disease as well as in patients with acute myocardial infarction (MI). Recent evidence also suggests that NHE inhibition may be conducive to attenuating the remodelling process after MI, independently of infarct size reduction, and attenuation of subsequent postinfarction heart failure. As such, inhibitors of NHE offer substantial promise for clinical development for attenuation of both acute responses to myocardial as well as chronic postinfarction responses resulting in the evolution to heart failure.

Increased Na(+)/H(+) exchanger isoform 1 activity in spontaneously hypertensive rats: lack of mutations within the coding region of NHE1

Enhanced Na(+)/H(+) exchange, measured as amiloride derivative-sensitive Na(+) and H(+) fluxes in cells with a preliminary acidified cytoplasm (Deltamu(H+)-induced Na(+)/H(+) exchange), is one of the most prominent intermediate phenotypes of altered vascular smooth muscle cell (VSMC) function in spontaneously hypertensive rats (SHR). Analysis of Na(+)/H(+) exchange in F(2) hybrids of SHR and normotensive rats seems to be the most appropriate approach in the search for the genetic determinants of abnormal activity of this carrier. However, the measurement of Deltamu(H+)-induced Na(+)/H(+) exchange is hardly appropriate for precise analysis of the carrier's activity in VSMC derived from several hundred F(2) hybrids. To overcome this problem, we compared the rate of (22)Na influx under baseline conditions and in Na(+)-loaded (ouabain-treated) VSMC. The dose-dependency of the rate of Deltamu(H+)-induced H(+) efflux as well as of (22)Na influx in control and ouabain-treated cells on ethylisopropylamiloride (EIPA) concentration were not different (K(0.5) approximately 0.3 microM), suggesting that these ion transport pathways are mediated by the same carrier. EIPA-sensitive (22)Na influx in Na(+)-loaded cells was approximately 6-fold higher than in ouabain-untreated VSMC and was increased by 50-70% in two different substrains of SHR. About the same increment of EIPA-sensitive (22)Na influx in Na(+)-loaded VSMC was observed in 5- to 6-week-old SHR (an age at which hypertension has not yet developed) as well as in stroke-prone SHR (SHRSP) with severe hypertension, indicating that the heightened activity of Na(+)/H(+) exchange is not a consequence of long-term blood pressure elevation. To examine whether or not the augmented activity of Na(+)/H(+) exchange in SHR is caused by mutation of NHE1, i.e. the only isoform of this carrier expressed in VSMC, we undertook single-stranded conformational polymorphism analysis of 23 NHE1 cDNA fragments from SHR and SHRSP and sequencing of the 456-2421 NHE1 cDNA fragment. This study did not reveal any mutation in the entire coding region of NHE1. The lack of mutation in the coding region of NHE1 indicates that the augmented activity of the ubiquitous Na(+)/H(+) exchanger in primary hypertension is caused by altered regulation of carrier turnover number or/and its plasma membrane content.

Genetic and biochemical determinants of abnormal monovalent ion transport in primary hypertension

Data obtained during the last two decades show that spontaneously hypertensive rats, an acceptable experimental model of primary human hypertension, possess increased activity of both ubiquitous and renal cell-specific isoforms of the Na+/H+ exchanger (NHE) and Na+-K+-2Cl- cotransporter. Abnormalities of these ion transporters have been found in patients suffering from essential hypertension. Recent genetic studies demonstrate that genes encoding the beta- and gamma-subunits of ENaC, a renal cell-specific isoform of the Na+-K+-2Cl- cotransporter, and alpha3-, alpha1-, and beta2-subunits of the Na+-K+ pump are localized within quantitative trait loci (QTL) for elevated blood pressure as well as for enhanced heart-to-body weight ratio, proteinuria, phosphate excretion, and stroke latency. On the basis of the homology of genome maps, several other genes encoding these transporters, as well as the Na+/H+ exchanger and Na+-K+-2Cl- cotransporter, can be predicted in QTL related to the pathogenesis of hypertension. However, despite their location within QTL, analysis of cDNA structure did not reveal any mutation in the coding region of the above-listed transporters in primary hypertension, with the exception of G276L substitution in the alpha1-Na+-K+ pump from Dahl salt-sensitive rats and a higher occurrence of T594M mutation of beta-ENaC in the black population with essential hypertension. These results suggest that, in contrast to Mendelian forms of hypertension, the altered activity of monovalent ion transporters in primary hypertension is caused by abnormalities of systems involved in the regulation of their expression and/or function. Further analysis of QTL in F2 hybrids of normotensive and hypertensive rats and in affected sibling pairs will allow mapping of genes causing abnormalities of these regulatory pathways.

Can we use erythrocytes for the study of the activity of the ubiquitous Na+/H+ exchanger (NHE-1) in essential hypertension?

Both Na+/Li+ countertransport and electrochemical proton gradient (delta mu(H+))-induced Na+ and H+ fluxes are increased in erythrocytes from patients with essential hypertension. It was assumed that these abnormalities are related to ubiquitous (housekeeping) forms of the Na+/H+ exchanger (NHE-1). To examine this hypothesis, we compared kinetic and regulatory properties of erythrocyte Na+/Li+ countertransport and delta mu(H+)-induced Na+ and H+ fluxes with data obtained for cloned isoforms of the Na+/H+ exchanger. In human erythrocytes, Na+/Li+ countertransport exhibited a hyperbolic dependence on [Na+]0 with a K0.5 of approximately 30 to 40 mmol/L. The activity of this carrier was increased by two-fold in the fraction of erythrocytes enriched with the old cells, was inhibited by 0.1 mmol/L phloretin, and was insensitive to both 1 mmol/L amiloride and ATP depletion. In contrast, delta mu(H+)-induced 22Na influx was exponentially increased at [Na+]0 > 60 mmol/L, was insensitive to phloretin, was partly decreased by both 1 mmol/L amiloride and ATP depletion, and was the same in total erythrocytes and in the old cells. The values of Na+/Li+ countertransport and delta mu(H+)-induced Na+ influx in erythrocytes from different species were not correlating and their ratio in human, rat, and rabbit erythrocytes was 10:1:170 and 1:5:1 for Na+/ Li+ countertransport and delta mu(H+)-induced Na+ influx, respectively. In contrast to the majority of nonepithelial cells and cells transfected with an ubiquitous isoform of Na+/H+ exchanger, both delta mu(H+)-induced Na+ influx and Na+/Li+ countertransport in human erythrocytes were completely insensitive to ethylisopropyl amiloride (20 micromol/L) and cell shrinkage. Thus, our data strongly suggest that human erythrocyte Na+/Li+ countertransport and delta mu(H+)-induced Na+/H+ exchange are mediated by the distinct transporters. Moreover, because the properties of these erythrocyte transporters and NHE-1 are different, it complicates the use of erythrocytes for the identification of the mechanism for activating the ubiquitous form of Na+/H+ exchanger in primary hypertension.

Calcium-induced activation of the rat vascular myocyte Na+/H+ exchanger isoform-1

An established intermediate phenotype of human hypertension and diabetic nephropathy is an elevation of Na+/H+ exchanger (NHE) activity, but the mechanism for this is unclear. This phenotype is maintained in vascular myocytes from the spontaneously hypertensive rat (SHR) compared with the normotensive Wistar Kyoto rat (WKY). Since intracellular calcium levels ([Ca2+]i) following agonist stimulation were elevated in cells from both hypertensive humans and SHR, we have examined the role of calcium-calmodulin (CaM) in the mechanism of increased NHE activity in vascular myocytes of SHR by determining the activity and phosphorylation state of NHE isoform-1 (NHE-1) in cells from SHR and WKY when [Ca2+]i was elevated by the ionophores A23187 or ionomycin. NHE activity was measured using fluorometry and NHE-1 phosphorylation by immunoprecipitating the exchanger from 32P-orthophosphate-labeled cells with a polyclonal NHE-1-specific antibody. The ionophore A23187 increased [Ca2+]i in both cell types to approximately 700 to 800 nmol x L(-1), and led to stimulation of NHE-1 activity only in WKY myocytes, with no effect on SHR cells. An inhibitor of CaM kinase II (KN-62) failed to abolish stimulation of NHE-1 by A23187 in WKY cells, and had no effect on unstimulated NHE-1 activity in both cell types. Ionomycin also elevated [Ca2+]i in both cell types to approximately 1,000 nmol x L(-1) and activated NHE-1 activity in only WKY cells. Activation of NHE-1 in WKY cells by an increased [Ca2+]i was not mediated by an increase in NHE-1 phosphorylation, whether in the presence or absence of KN-62. The elevated NHE-1 phosphorylation in SHR cells was not affected by elevated [Ca2+]i or KN-62. Calmodulin-agarose beads bound NHE-1 extracted from SHR cells to a lesser extent than that from WKY cells. We conclude that calcium-induced NHE-1 activation in WKY cells was not mediated by CaM kinase II. The elevated NHE-1 activity and phosphorylation of SHR cells was not further modulated by increased [Ca2+]i, and was also independent of CaM kinase II. Non-phosphorylation-dependent mechanisms of activation of NHE-1 may therefore be responsible for alterations of NHE-1 activity in these cells, such as the direct binding of CaM to NHE-1. This direct binding of CaM to NHE-1 may be impaired in SHR compared with WKY cells.

Enhanced mitogen-activated protein kinase activity and phosphorylation of the Na+/H+ exchanger isoform-1 of human lymphoblasts in hypertension

Increased activity of the Na+/H+ exchanger isoform-1 (NHE-1) is recognized as an intermediate phenotype for hypertension, but the basis for this association is unclear. We have previously demonstrated an increased phosphorylation of NHE-1 in lymphoblasts from hypertensives that was associated with increased cell proliferation. Due to the central importance of mitogen-activated protein kinases (MAPKs) in signaling cascades transducing responses from extracellular growth factors and hormones, we examined the activity of this kinase in a specific peptide phosphorylation assay. Cells from hypertensives showed a significant twofold enhancement of MAPK activity (P < .001). This was not associated with any increase in p42mapk and p44mapk protein content. There was no significant increase in the level of tyrosine phosphorylation of MAPK in cells from hypertensives. MAPK activity was correlated with NHE-1 activity (r(s) = .55, P < .01) and phosphorylation (r(s) = .51, P < .02). These findings suggest that the increased cell proliferation rate, NHE-1 activity, and phosphorylation of lymphoblasts from hypertensives may be associated with enhanced MAPK activity, suggesting upregulation of this signaling pathway in hypertension.